The invention relates to a process and an apparatus for the purification and/or treatment of suspensions, which contain at least one or more precious metals and/or rare earths or other elements in the form of a precipitate or sediment.
During the manufacture of pure precious metals and/or rare earths and/or other elements of the Periodic System or their salts, the precious metal or the rare earth or the element is dissolved in a corresponding suitable solvent. In addition to the specific element or precious metal, other elements, which are similar regarding the dissolving behaviour, normally also dissolved and this can constitute impurities.
By adding precipitating agents specifically suited to the respective element or regulation of the solution equilibrium with a corresponding buffer solution or pH-shift, it is possible to selectively precipitate from the solution the desired individual elements or precious metals or rare earths. This results in a suspension, which contains the deposited precipitate or the solid as well as the materials still remaining in the solution.
To obtain the desired precious metal and/or element in purest form, it is necessary to purify the desired deposited precipitate present in the suspension.
According to the state of the art, the washing and purification of such precipitated precious metals from the suspension occur in so-called glove-boxes, in which the liquid phase moves through a filter medium by application of a vacuum, which becomes quickly covered and impermeable due to the extreme fine solids: To prevent this the solution is stirred manually. To wash about 10 kg of platinum hydroxide with such a process about 10 to 15 working days are needed.
A further disadvantage of such processes with glove boxes is the burden on the environment.
Object of the invention is thus to provide a process and an apparatus by means of which these disadvantages can be prevented. Particularly, the washing should occur in a closed-system in the process in accordance with the invention, least possible washing liquid is required and the washing of quantities in the region of for example 10 kg of platinum occurs in the shortest possible time, preferably in less than one working day.
According to the invention, a process for the purification and/or treatment of suspensions, which include at least one or more precious metals and/or rare earths or other elements in the form of a precipitate or solid and/or also in dissolved form, includes the following steps:
1. concentrating or thickening the precipitate or solid in the suspension by removal of the liquid phase by means of a membrane filter apparatus, whereby dissolved impurities and/or precious materials are discharged with filtrate;
2. washing of the concentrated or thickened suspension by addition of a washing liquid and removal of filtrate by means of the membrane filter apparatus, whereby the concentration or thickening of the solids in the suspension does not or changes only slightly during washing and the concentration or thickening of the dissolved impurities and/or precious materials still present in the suspension are reduced to a pre-determined value; and
3. complete drainage of the membrane filter apparatus to provide, in purified form, solid and/or purified precipitates of the precious metal(s) and/or rare earths or other elements remaining in residual suspension for further treatment and/or further application.
The invention therefore proposes that the solids contained in the suspension or the precipitate initially are concentrated or thickened by removing the liquid phase of the suspension by means of a membrane filter apparatus, whereby the dissolved impurities and/or precious materials are carried out with the filtrate. By means of the concentration or the thickening, the concentration of the solid or precipitate is increased in the suspension. After reaching the maximum possible reduction of the starting volume, the suspension is washed by addition of a washing liquid and the removal of the filtrate by means of the membrane filter apparatus. Thereby the dissolved impurities and/or precious metals or elements remaining in the solution are reduced to a predetermined value. To improve the efficiency of the washing the liquid can also contain additives or buffer solutions.
After the washing of the suspension containing the insoluble precipitate, the membrane plant is completely drained. The desired product, i.e. the purified precipitate of the precious metal(s) and/or rare earths or other elements are then available in highest purified form for further application or further processing.
The process according to the invention is suitable, in particular, for recovering one or more of the following elements: gold, platinum, rhodium, palladium, ruthenium, iridium, osmium, cobalt, europium, lanthanum, germanium, gallium, cerium, tantalum, niobium, selenium, tellurium, cadmium, bismuth, beryllium, uranium, manganese, in highest purified form as precipitate or in dissolved form.
The washing liquids are appropriately chosen according to the specific chemical behaviour of the element to be scrubbed out.
In particular, the washing liquid may contain acetic acid and/or ammonium nitrate and/or sodium carbonate or sodium bicarbonate and/or hydrochloric acid and/or nitric acid and/or aqua regia.
In a preferred embodiment form, it is provided that the concentration of the impurities still dissolved in the liquid phase of the suspension after scrubbing, lies in the range of 50 mg per liter to 0.001 mg per liter.
The process temperature of both the concentrating and also the washing by means of the membrane filter apparatus lies in the range of 5xc2x0 C. to 150xc2x0 C., during the treatment of rhodium hydroxide preferably between 75xc2x0 C. and 85xc2x0 C. and during the treatment of platinum hydroxide preferably between 40xc2x0 C. and 50xc2x0 C.
Preferably, the membrane fitter apparatus used for the concentrating and washing, is operated according to a tangential flow filtration process, i.e. the membrane, which serves as fitter body, is over flowed over by a continuous speed tangential to the membrane surface, so that deposits of materials on the membrane surface are substantially prevented by the tangential flushing.
Impurities, which deposit in the pores despite the flowing over, can be removed from the membrane surface by the introduction of a back flush impulse during the operation. Such a back flush impulse removes large impurities from the surface of the membrane by mechanical separation.
The nominal pore size of the membranes of the membrane filter apparatus may be in the range between 5 nm and 1,4 xcexcm, whereby specially preferred are membranes with a separating size of 50 to 100 nm.
Membranes of ceramics, polymeric materials, metal or inorganic materials may be used as membranes in the membrane filter plant. Especially preferred is the use of ceramic membranes or membranes of other inorganic materials due to their high chemical and physical stabilities.
The operating pressure in the membrane filter apparatus or its modules lies in the range of 0.5 to 50 bar, preferably in the range of 2 to 6 bar.
Also according to the invention there is provided an apparatus for the operation of a process as set out herein, characterised thereby, that the apparatus includes:
at least one pre-tank, and
at least a membrane filter apparatus.
The apparatus for the operation of the process is therefore characterised thereby, that it includes at least one storage apparatus or pre-tank, in which the suspension to be treated is introduced and thickened, as well as a membrane filter apparatus with which the concentrating and washing can take place. A very decisive advantage lies therein that the filter apparatus in an embodiment of the invention is constructed in such a way, that it can be completely drained by gravity or positive gas pressure. This ensures that no residues remain in the pump sump after emptying and a product in highest purified form can be removed from the plant as slurry/suspension.
For impurities of the membrane filter apparatus, which cannot be removed by mechanical back pulse, it is advantageous if the membrane filter apparatus is provided with a cleaning apparatus, which makes it possible to introduce cleaning liquid to the membrane module(s), which chemically dissolves the absorbate layers deposited on the membranes. Chemical cleaning substances for this are, for example, acetic acid or hydrochloric acid or nitric acid or aqua regia or HF-acids. For example, HF-acids are used to dissolve polymeric silicic acid.
Concerning the different purification processes of membrane modules in membrane filter apparatus, reference is made to WO 99/42203, the contents of which is considered to be fully incorporated in the present application.
In a first embodiment form, the membrane filter apparatus can consist of one membrane module. Alternatively, the membrane apparatus can also include a number of membrane modules, which are located either in series or parallel. These membrane modules can either be back flushed together or individually.
In a further embodiment of the invention, it is provided that the membrane filter apparatus consists of several in series arranged membrane loops, which can be removed individually from the operation and/or be back flushed individually or together.